In reviewing recent developments in the process mineralogy of gold it is convenient to distinguish between visible (>0.1 pm) and invisible gold (<0.1 m). automatic electron-beam based systems have been developed for the location and characterisation of visible gold minerals>1 - 2 pm in size in polished section; these can scan the surface of a polished section, identifying the gold minerals, recording co-ordinates of the grains for subsequent examination, and making measurements of areas and sizes of the grains. However, problems of sample reliresentivity in polished section mean that such data may not give good quantification of gold mineral proportions and liberation/locking characteristics and other methods are required for this (amalgamation, cyanidation, ultra-fine milling and cyanidation). Characterisation of invisible gold in bulk material (eg a sulphide concentrate) can be done by mineral separation, ultra-fine milling and cyanidation or by quantitative point analyses of a large number of mineral grains. Analytical methods include electron microprobe analysis (detection limit normally -200 ppm but -10 ppm by using special techniques), ion microprobe analysis (secondary ion mass spectrometry - SIMS; detection limit -0.2 ppm) or proton microprobe analysis (PIXE; detection limit <5 - 40 ppm) and there is now evidence of invisible gold in a wide range of sulphide minerals and some non-sulphide minerals._x000d_>
High-resolution transmission electron microscopy can be used to detect particulate invisible gold and Mossbauer spectroscopy has been used to show the presence of chemically combined invisible gold in sulphides. Experimental work on the adsorption of gold from solution by minerals indicates that the common clay minerals (illite, kaolinite, montmorillonite) do not adsorb gold from cyanide solution to any significant extent but heavy minerals (including pyrite) can show gold adsorption. Four methods for quantifying the deportment of gold in ores and mill products are described. These are the Amdel method (Henley, 1989b), the method of Chryssoulis and Cabri (1990), diagnostic leaching (Lorenzen and Tumilty, 1992) and the use of bacterial leaching (Katsikaros, 1991)._x000D_
The Amdel method and the method of Chryssoulis and Cabri both use a combination of metallurgical and mineralogical techniques to give a comprehensive picture of the gold distribution among different mineralogical sites. Diagnostic leaching can show the proportion of gold associated with different minerals but generally does not characterise the gold fuither (eg whether visible or invisible); it has some limitations where cyanide-soluble minerals such as chalcocite are present and there appears to be a need for research into the mineralogical changes accompanying the leaching stages. Bacterial leaching and cyanidation can be used to quantify the distribution of gold among different sulphides, the main examples studied to-date being mixtures of pyrite and Arsenopyrite.